The present invention relates generally to energy conservation apparatus for use in heating, ventilating and air conditioning (HVAC) systems. More particularly, the invention relates to total enthalpy air to air rotary energy exchangers, also known as total heat wheels and specifically to novel total heat exchange media employed in such wheels.
Rotary air to air energy exchangers are used in HVAC systems to recover both sensible energy (temperature) and latent energy (moisture) from an exhaust air stream and then exchange these with an incoming air supply stream. The ability to recover the latent energy represented by the moisture is of significant interest since it dehumidifies the outdoor air during the cooling cycle, and humidifies the outdoor air during the heating cycle so as to reduce the energy demands required to condition outdoor air by as much as 95 percent. Frost formation and subsequent freezing and plugging problems encountered in sensible heat exchangers in winter operations are also eliminated in most cases through the use of total energy exchangers, thus, making them of particular interest.
Heretofore, in the evolution of such wheels, the total heat exchange media was formed of a substrate of paper or asbestos material, impregnated or coated with a desiccant, typically a water soluble salt such as lithium chloride. Later, such enthalpy wheels contained total heat exchange media having an aluminum foil substrate with a surface coating of a desiccant material consisting of a dry film of silica gel, an oxidized surface or a separate coating of aluminum oxide. These later aluminum wheels exhibit greater strength and durability than the prior paper and asbestos media and also have the advantage in some cases of being capable of being washed with water and/or steam without harming the desiccant coating.
The sensible and latent heat exchange media used in the wheels are generally in the form of a matrix that provides air passages therethrough through which an air stream can flow. The matrices can take a variety of forms, such as a fibrous mesh or honeycomb. One type of honeycomb matrix is formed of a plurality of spaced, substantially parallel layers of a sheet material, particularly alternating layers of a corrugated sheet material and a flat sheet material. In the latter case, the corrugations are generally parallel and provide a plurality of axially extending passageways extending along the depth of the wheel.
Such a total energy recovery wheel, when placed between two air streams in counterflow, will enable sensible heat to be transferred between the two air streams. The air stream with the higher temperature will heat the sensible heat exchange material of the sensible and latent heat exchange media of the slowly rotating wheel which, in turn, heats the lower temperature air stream. Further, the wheel transfers the latent heat between two air streams of different absolute humidities. The desiccant portion of the sensible and latent heat exchange media of the wheel adsorbs moisture from the air stream of higher absolute humidity until reaching equilibrium with the air stream of higher absolute humidity and, as a result of the vapor pressure differential between the desiccant and the air stream of lower absolute humidity, will release the moisture to the air stream of a lower absolute humidity until reaching equilibrium with the air stream of lower absolute humidity. Absolute humidity is defined as pounds of moisture per pound of dry air and is distinguishable from relative humidity, which is defined as the ratio of the absolute humidity to the maximum possible density of water vapor in the air at a given temperature.
Thus, in the cooling mode, the energy recovery wheel cools and dries hot, humid incoming air by absorbing sensible heat and moisture from the incoming air. The sensible heat and moisture absorbed by the sensible and latent heat exchange media of the wheel are then transferred to the cool, dry outgoing air. As a result, the energy necessary to further cool and dehumidify the incoming air to the desired temperature and humidity is reduced.
In the heating mode, the energy recovery wheel heats and humidifies cool, dry incoming air by absorbing sensible heat and moisture from the warm, humid outgoing air. The sensible heat and moisture absorbed by the sensible and latent heat exchange media of the wheel are then transferred to the cool, dry incoming air. As a result, the energy necessary to further heat and humidify the incoming air to the desired temperature and humidity is reduced.
One problem associated with total energy recovery wheels arises from the fact that, generally, a typical wheel rotates at a rate of about twenty revolutions per minute. Thus, the wheel is in contact with each air stream for only about 1.5 seconds per revolution. The sensible heat exchange material and the desiccant must, therefore, be capable of absorbing the sensible and latent heat from and releasing the absorbed sensible and latent heat to the respective air streams very quickly.
A second problem with the use of such total energy recovery wheels is that desiccants heretofore used in the sensible and latent heat exchange media often adsorb and transfer contaminants found in the exhaust air stream along with the moisture. It should be noted that the most important reason for continuously exhausting air from an enclosed space and replacing it with fresh air is to remove air borne contaminants from the air in the enclosed space. Such undesirable contaminants include ammonia, hydrocarbons from solvents, carbon monoxide, nitrogen dioxide and sulfur dioxide. Desiccants such as activated alumina (Al.sub.2 O.sub.3) and silica gel have a very wide pore size distribution, 8 Angstroms to 70 Angstroms and 8 Angstroms to 100 Angstroms, respectively. Oxidized aluminum surfaces have an even wider pore size distribution. This wide pore size distribution allows the desiccant to adsorb air borne contaminants as well as moisture from the air stream. Desiccants such as lithium chloride deliquesce to form an aqueous desiccant solution which absorbs all water soluble contaminants. These contaminants are then released back into the incoming air stream along with the moisture. Contaminants also use adsorptive capacity of the desiccant that would otherwise adsorb moisture.
Thus an object of the invention is to develop a sensible and latent heat exchange media that is considerably more efficient and faster acting than those currently available.
A further object of the invention is to develop a sensible and latent heat exchange media for a total energy recovery wheel that will adsorb the moisture from an air stream, but will not adsorb contaminants present in the exhaust air stream so that the contaminants will be purged along with the exhaust air stream rather than be desorbed into and recirculated with the incoming fresh air stream.